The disclosed invention relates to polyisocyanate-impregnated lignocellulosic substrates. More particularly, the invention is a polyisocyanate-impregnated lignocellulosic substrate that is able to withstand moisture and displays a resistance to fungus and insects. The polymerized substrate may be used for doors, door parts and the like.
Hollow core doors are used principally in interior applications. A hollow core door may be a flush door, that is one flat or planar, with or without molded surfaces, on both major surfaces. The skins used for flush doors are relatively inexpensive, but they do not provide the aesthetic features and physical properties sometimes required by consumers. Hollow core doors manufactured from medium and high density fiberboard skins are not typically used in exterior applications, due to problems associated with moisture absorption and the resultant swelling of the cellulosic fibers.
Many hollow core doors are made from door skins, rails and stiles formed from wood and/or composite materials. These wood composite materials may include particle board, flake board, hard board and medium density fiber board (“MDF”). The wood composites utilize a resin binder, which is frequently a thermal setting resin, in order to maintain the wood fibers forming the composite in solid form. The wood composites are not moisture impervious, so doors utilizing such composites may not be suitable for exterior applications. Should the composite material absorb moisture, whether in liquid or gas form, the door components may swell and the door become distorted. Fiberglass and steel doors do not have the same moisture absorbing tendency, and hence are more frequently used for exterior applications.
The use of urea-formaldehyde or phenol-formaldehyde resins as binder material in wood composites is also known in the art. After polymerization of impregnated wood composites as described above, these resins tend to strengthen composite door materials by forming a three-dimensional cross-linked structure in and around the wood fibers. However, they do not form chemical bonds to the cellulose molecules of the lignocellulosic fibers, but instead they merely encapsulate the wood fibers in a physical net of cross-linked resin. Generally speaking, physical bonds, such as those just described, are much weaker than chemical bonds. Phenol-formaldehyde binder is additionally unsatisfactory because its cross-linking reaction proceeds at a relatively slow rate and requires a temperature in excess of 350° F.
Resin-impregnated substrates have in the past been disclosed but their manufacture has been undesirable because they required the use of a solvent or vapor recovery system, long cure times, and relatively high manufacturing costs due to oven curing. These efforts involving dry curing or curing that does not take place by application of a heated liquid, have also resulted in a surface appearance that is too glossy, cracked, marred, and/or is otherwise aesthetically displeasing. First, both fine and coarse, broken or unbroken bubbles are formed on the surface due to CO2 escaping through a film of resin formed on the substrate surface, resulting in a rough, pitted and generally marred appearance. Second, the surface film of cured resin cures to a high gloss finish. Third, the resin film tends to pool and run before curing is completed, resulting in streaks, runs, and drips on the substrate surface.
Attempts to remove the high gloss, rough, pitted and marred surface film from the polyisocyanate-impregnated substrate have been unsuccessful because such efforts leave a rough, matte, highly textured surface having a wholly unsuitable aesthetic appearance. Additionally, if the substrates are molded or otherwise configured into a three dimensional pattern before impregnation, as is done for molded door skins, the three dimensional design or pattern on the cured, impregnated substrate is ruined if the oven-cured surface film is removed. This is because the fine details, lines, curves, ridges, and other three dimensional patterns are scraped, sanded, gouged, or otherwise worn down and marred as the surface film is removed.
Those skilled in the art will recognize that there is a need for a polyisocyanate-impregnated lignocellulosic substrate exhibiting suitable strength and water resistance so that it may be used for exterior applications. Yet a further need in the art is a method of manufacturing such polyisocyanate-impregnated lignocellulosic substrates, but without requiring expensive oven curing or additional surface processing. The disclosed invention meets these and other needs in the art.